Methods of light microscopy, transmission electron microscopy and immunohistochemistry were used to study the morphogenesis of elastic fibers in nuchal ligament, aorta and lung of fetal, neonatal and postnatal sheep. Developing elastic fibers were found to consist of amorphous components (elastin) and surrounding microfibrils. Antielastin antibody reacted with the microfibrils and with the amorphous components. Microfibrils appear to serve as sites for aggregation of elastin during elastogenesis. The process of maturation of elastic fibers can be evaluated by the use of appropriate staining methods, including the periodic acid-silver methenamine technique and the tannic acid method of Kajikawa.